1. Field of the Invention
The present invention relates to the field of DC motors, and more particularly to DC motor speed monitoring and diagnostics.
2. Description of the Related Art
Direct current (DC) motors are used for numerous applications. In many of those applications, it is desirable to know the speed at which the motor is running. Knowledge of the speed of the motor can help facilitate closed-loop control of the speed of the motor. Further, knowledge of the speed of the motor can be used to help detect motor fault conditions. For example, a motor can become mechanically overloaded. Such mechanical overloading can reduce the speed of the motor such that it does not operate as designed, as well as cause the motor to draw electrical currents above intended values.
One way to measure speed of a motor is to monitor the commutator signal of the motor. The frequency of this signal is proportional to the speed of a DC motor having brushes. Further, the frequency of the signal is proportional in almost all cases to the speed of a brushless DC motor. However, monitoring the commutator signal requires that an additional terminal be provided at the motor (as an output for the commutator signal) and at the controller for the motor (as an input for monitoring the commutator signal). Such increased terminal count increases the cost and decreases the reliability of the system. In some applications, such as in automobiles, cost constraints and reliability requirements are particularly stringent. Especially in such applications, a speed monitoring system for a DC motor which would not require the additional terminals herein described would provide cost and reliability advantages.